This invention relates to a plugging device comprising a pump for introducing liquid metal from a main pipe, a cooler for cooling the liquid metal introduced by the pump, a plugging orifice having an orifice hole through which the cooled liquid metal is passed, whereby impurities dissolved in the liquid metal are precipitated mainly at the orifice hole to increase the flow resistance, a flow meter for measuring the flow rate of the liquid metal, a thermometer for measuring the temperature of the liquid metal flowing through the plugging orifice, and a control block for controlling the cooling capability of the cooler in response to flow rate signal delivered from the flow meter, so that the plugging temperature of the liquid metal is determined from the temperature measured by the thermometer.
The aforesaid type of plugging device, which has been widely used, involves many defects to be improved. Such defects will now be described with respect to a plugging device for measuring the plugging temperature of liquid sodium.
In various plants or sodium testing loops employing, for example, liquid sodium as a coolant, it is necessary to control impurities dissolved in the liquid sodium within a predetermined concentration range, in order to prevent corrosion of structural material and abrasion of sliding parts or to enable early-stage detection of water leak in a boiler using liquid sodium. These impurities may include Na.sub.2 O, NaH, etc. dissolved in liquid sodium. Generally, a well-known cold-trap device may be used for removing excessive impurities dissolved in liquid sodium, while a plugging device may usually be employed to detect the concentration of the impurities in the liquid sodium.
The conventional cold-trap device is a device to cool a liquid metal to a desired temperature and remove impurities precipitated at such temperature. In the plugging device, a fluid metal flowing through a passage with an orifice or an orifice passage is cooled to precipitate impurities dissolved in the liquid metal at the orifice or plugging orifice. Then the reduction of the flow rate of the liquid metal caused by such precipitation and the temperature of the liquid metal at the time of such reduction, i.e. plugging temperature, are measured, and the concentration of the dissolved impurities is determined according to the known relationship between the plugging temperature and the impurity concentration.
There are three systems to operate the plugging device; manual, automatic oscillatory, and automatic continuous operation systems. There will be described herein the generally used second and third systems.
In an automatic oscillatory operation of the device, the liquid metal is diverted from a process to be monitored by means of a suitable pump, and returned to the process through an orifice passage including a flow meter, cooler and plugging orifice. In the meantime, the flow rate and temperature of the liquid metal passing through the plugging orifice are measured by the flow meter and a theremometer disposed near the plugging orifice, respectively.
In measuring the plugging temperature by using the plugging device, the cooler is driven in response to a measured value of flow rate provided by the flow meter, and the temperature of the liquid metal passing through the plugging orifice is continuously reciprocatively varied between upper and lower limits by a suitable automatic control system. The plugging temperature to be determined may be found between the maximum and minimum values of the varying temperature. In this case, the plugging orifice is required to have its flow resistance changed susceptibly with temperature changes of the liquid metal for the high-accuracy, speedy measurement of the plugging temperature. Further needed is stability in the operation of the control system, as well as reduced time constant therefor. The time constant may effectively be reduced by diminishing the thermal capacity of the control system.
In order to fulfill the aforesaid requirements, various improvements have been made on the prior art plugging devices. One such improvement is the development of an annular-linear center-return electromagnetic pump with reduced size and simple pipe arrangement to be used in place of the conventional flat-linear, helical or AC conductive electromagnetic pump. Another improvement is the use of a high-sensitivity electromagnetic flow meter. Moreover, in order to improve the sensitivity of the plugging orifice and to prevent clogging thereof, there have been proposed and employed a valvelike plugging orifice with adjustable cross-sectional area of passage, plugging orifice formed of a plurality of restrictions, plugging orifice with an irregular hole.
With all these improvements, the prior art plugging devices cannot make high-accuracy measurement, requiring long measuring time and failing to secure stability in control. This is attributable to the fact that none of the plugging orifices developed so far has enough sensitivity and that the control system for the cooler is unsuitable.
Heretofore, an air flow from a blower delivering a fixed volume of air has been restricted to a desired volume, and then introduced into a cooling portion of the cooler. The air flow is restricted by rotating a damper attached to the blower correspondingly to the flow rate of the liquid metal. In such cooling system, the volume of air delivered from the blower through the damper is not zero even though the opening of the damper is zero. Accordingly, the cooling capability of the cooler cannot be reduced to zero. Further, the change of the volume of air or the cooling capability is larger as compared with the change of the damper opening while the damper opening is relatively small, whereas the change of the cooling capability becomes relatively smaller as the damper opening is increased.
The above-mentioned cooler control by means of the damper is not suited for the automatic oscillatory operation of the plugging device, exhibiting low accuracy for the plugging temperature measurement. This is the following necessity because the fact that prior art device is lacking. The necessity is, like the case of the manual control system, to maintain the cooling speed of the liquid metal substantially at a fixed level determined by the properties of the metal for high-accuracy measurement of plugging temperature by the automatic oscillatory operation. When using liquid sodium, for example, the cooling speed may be selected in a range from 3.degree. to 10.degree. C./min. preferably at 5.degree. C./min. In order to obtain such cooling speed, high cooling capability and hence large volume of air are required where the concentration of impurities dissolved in the liquid metal and the plugging temperature are relatively low, whereas low cooling capability and hence small volume of air are needed where the impurity concentration and the plugging temperature are relatively high. According to the control system employing the damper, however, it is impossible to make high-accuracy measurement of the plugging temperature of liquid metal with high impurity concentration for the following reasons. That is, even though not required, the volume of air cannot be reduced entirely to zero. If a small volume air is required, it is necessary to use a low damper opening range in which the volume of air is changed largely with a small change of the damper opening, so that it is difficult to obtain a desired volume of air by finely controlling the damper opening. Also, where an extremely small volume of air is needed, the measurement will unavoidably be made with an unnecessarily large volume of air and hence an unnecessarily high cooling speed.
Moreover, where the aforementioned damper control system is applied to the automatic continuous operation, the flow rate of the liquid metal passing through the plugging orifice is lowered since the volume of air cannot entirely be reduced to zero. Therefore, if the damper opening is reduced to zero, the temperature of the liquid metal cannot be increased due to the remaining air flow, leaving the impurities to be continuously precipitated at the plugging orifice, thereby causing clogging of the orifice.
Other difficulties caused in the automatic continuous operation may be as follows. In general, the liquid metal is required to be cooled positively where the impurity concentration and the plugging temperature are both low, so that it is necessary to cool the liquid metal by a large volume of air with increased damper opening. With such low impurity concentration, however, the amount of impurities precipited by a drop of liquid metal temperature is relatively small, and thence the change of the flow rate of the liquid metal is also relatively small. Accordingly, in the conventional automatic continuous operation where the flow rate value of the liquid metal is automatically controlled so as to coincide with a predetermined value by changing the liquid metal temperature in accordance with the deviation of the flow rate, it is advisable largely to change the flow of air supplied through the damper as compared with the flow rate deviation, for the stability of the automatic control. If the impurity concentration is high, on the other hand, the amount of impurities precipitated by the temperature drop is relatively large, and therefore the change of the flow rate is also relatively large. Accordingly, in the automatic continuous operation, it is to be desired for the fineness of the automatic control that the change of the volume of air from the damper be relatively small as compared with the flow rate deviation. Meanwhile, in the conventional damper control system to change the damper angle in proportion to the flow rate deviation, the change of the volume of air caused by the flow rate change of liquid metal is the contrary of desired one, so that the flow rate of the liquid metal may vary from the predetermined value, failing to secure accurate measurement of the plugging temperature.
Thus, the prior art system to cool the liquid metal by means of the air flow delivered through the damper is not suitable for either of the automatic oscillatory and continuous operations of the plugging device.